When selecting a network for wireless service, a cell phone that supports multiple radio access technologies (RATs) needs to select both a public land mobile network (PLMN) and a radio access technology (RAT) while performing a search for a serving cell. To facilitate such a PLMN/RAT selection, a universal Subscriber Identity Module (USIM) card of the cell phone may include three data files, that is, (1) a Home PLMN (HPLMN) Selector with Access Technology (EF-HPLMNwAcT) data file, (2) a User Controlled PLMN Selector with Access Technology (EF-PLMNwAcT) data file, and (3) an Operator Controlled PLMN Selector with Access Technology (EF-OPLMNwAcT) data file. Pursuant to Technical Specification (TS) 31.102 of the 3GPP (Third Generation Partnership Project) standards, each entry in these data files defines a PLMN/RAT(s) combination where multiple RATs specified in a single entry indicate that those RATs are equally preferred access technologies for the listed PLMN. The EF-PLMNwAcT and EF-OPLMNwAcT data files provide the preferred roaming partners and access technologies when the subscriber is outside of HPLMN coverage.
According to TS 23.122 of the 3GPP standards, the PLMN/RAT entries in the EF-PLMNwAcT and EF-OPLMNwAcT data files shall be considered for selection in priority order. For example, if PLMN ‘A’ and PLMN ‘B’ both are roaming partners of a subscriber's HPLMN, but PLMN A and an associated RAT ‘X’ (PLMN-A/RAT-X) is listed before PLMN B and an associated RAT ‘Y’ (PLMN-B/RAT-Y) in the EF-PLMNwAcT or EF-OPLMNwAcT data files maintained in the USIM card of the subscriber's cell phone, then PLMN-A/RAT-X will be selected when it is detected, even if PLMN-B/RAT-Y is also present. It may be noted that the EF-HPLMNwAcT data file, however, only defines the RAT preference for the subscriber's HPLMN and all equivalent HPLMNs and the PLMN code of each entry in EF-HPLMNwAcT data file is not used, as per TS 23.122.
As a result, if an old Subscriber Identity Module (SIM) card is used in a multi-mode cell phone that supports a new RAT not listed in the old SIM card, then that new RAT may never be utilized in certain scenarios because the new RAT has never been programmed in any preferred list file entry of the old SIM card. For example, if a subscriber uses the SIM card from his or her old UMTS (Universal Mobile Telecommunications System) cell phone in a new E-UTRA (Evolved UMTS Terrestrial Radio Access) capable multi-mode cell phone, and the subscriber is located outside of the HPLMN's coverage area and in an area where the most preferred roaming partner network only provides E-UTRA coverage, then the new multi-mode cell phone may never be able to camp on that roaming partner's E-UTRA network. Specifically, if PLMN-A/UTRA is listed before PLMN-B/UTRA in the EF-PLMNwAcT data file or the EF-OPLMNwAcT data file of the old SIM card, and the subscriber is within an area where only PLMN-A/E-UTRA and PLMN-B/UTRA are present, then the new multi-mode cell phone will stay on PLMN-B/UTRA even though PLMN-A is a more preferable roaming partner and even though the new multi-mode cell phone does support E-UTRA access technology.
This problem cannot be resolved by performing periodic HPPLMN (Higher Priority PLMN) scans to find higher priority PLMNs because, based on the existing USIM preferred list information, PLMN-A/E-UTRA is not a higher priority PLMN/RAT combination than PLMN-B/UTRA. Also, this problem cannot be resolved by performing Inter-RAT reselections within PLMN B because even if the new multi-mode cell phone could reselect to PLMN-B/E-UTRA, the multi-mode cell phone still will not go to PLMN-A/E-UTRA.
One of ordinary skill in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Also, common and well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.